Tire pressure monitoring systems are traditionally used in automotive applications to monitor the inflation pressure of vehicle tires and to warn the driver in case of abnormal inflation.
Standard TPMS modules are valve-based, i.e. mounted on the valve and thus fixed to the rim. In contrast to valve-based TPMS modules, tire-mounted modules are mounted on the inner liner of the tire (mounted in the cavity of each tire).
For direct TPMS, modules—comprising at least of a pressure sensor, control logic, a radio frequency (RF) transmitter and a source for electrical energy—are mounted on the tire. Each module measures the inflation pressure and transmits this value together with module identification (ID) via RF to the electronic control unit (ECU) in the vehicle.
Localization describes the process of mapping the module IDs to the individual tires. This enables the TPMS to attribute the received inflation pressures to the individual tires and to signal which tire is abnormally inflated. Modern ECUs can perform tire localization automatically by synchronizing ABS (antilock braking system) wheel speed sensors with the angular positions of the TPMS modules (angular position sensing, APS). TPMS modules may measure their respective angular positions and transmit this information to the ECU for that purpose.
One way to do so with valve-based TPMS modules is by inferring the angular position from the direction of the earth's gravity, which is measured with accelerometers. The accelerations acting on such modules comprise mainly of the centrifugal acceleration due to the spinning wheel, mechanical vibrations, and the earth's gravity.
In contrast to valve-based TPMS modules, tire-mounted TPMS modules are mounted on the inner liner of the tire. As the tire spins during vehicle movement, such tire-mounted TPMS modules follow roughly a trajectory corresponding to the tire's circumference. In the vehicle-frame, i.e. a coordinate system which is fixed to the vehicle, the tires' circumferences and thus the trajectories resemble flattened circles, where the flat is determined by the contact patch (footprint) between the tire and the ground. Tire-mounted TPMS modules are thus subject to fundamentally different acceleration waveforms than valve-based TPMS modules. The gravity-based APS methods of valve-based modules are therefore inapplicable for tire-mounted modules.